--- a/src/hotspot/share/gc/parallel/psCardTable.cpp	Fri Aug 16 09:18:29 2019 +0200
+++ b/src/hotspot/share/gc/parallel/psCardTable.cpp	Fri Aug 16 09:18:32 2019 +0200
@@ -128,6 +128,38 @@
 // when the space is empty, fix the calculation of
 // end_card to allow sp_top == sp->bottom().
 
+// The generation (old gen) is divided into slices, which are further
+// subdivided into stripes, with one stripe per GC thread. The size of
+// a stripe is a constant, ssize.
+//
+//      +===============+        slice 0
+//      |  stripe 0     |
+//      +---------------+
+//      |  stripe 1     |
+//      +---------------+
+//      |  stripe 2     |
+//      +---------------+
+//      |  stripe 3     |
+//      +===============+        slice 1
+//      |  stripe 0     |
+//      +---------------+
+//      |  stripe 1     |
+//      +---------------+
+//      |  stripe 2     |
+//      +---------------+
+//      |  stripe 3     |
+//      +===============+        slice 2
+//      ...
+//
+// In this case there are 4 threads, so 4 stripes.  A GC thread first works on
+// its stripe within slice 0 and then moves to its stripe in the next slice
+// until it has exceeded the top of the generation.  The distance to stripe in
+// the next slice is calculated based on the number of stripes.  The next
+// stripe is at ssize * number_of_stripes (= slice_stride)..  So after
+// finishing stripe 0 in slice 0, the thread finds the stripe 0 in slice1 by
+// adding slice_stride to the start of stripe 0 in slice 0 to get to the start
+// of stride 0 in slice 1.
+
 void PSCardTable::scavenge_contents_parallel(ObjectStartArray* start_array,
                                              MutableSpace* sp,
                                              HeapWord* space_top,